Currently, various vibration models have been proposed and put to practical use. In general, vibration characteristics depend on the loaded mass and the input. It is considered that there is a correlation between the loaded mass and the curvature of load-deflection characteristics, and between the input and the hysteresis of load-deflection characteristics.
For example, in suspension systems for automotive vehicles, the points for adjusting ride comfort, such as the spring constant of the suspension systems, are the road surface condition, the control stability, and the impedance (or the difference thereof condition. For the optimization under all the conditions, the active control is needed. Annoying road driving or high-speed driving result in significant differences in a low frequency and high amplitude region. In the case where the damping force is low, the transmissibility of deflection will increase, and the resonance frequency will shift to the lower frequency region. In order to increase the damping force, it is necessary to increase the damping ratio of a damper, or decrease the spring constant. Therefore, conventional passive vibration models have a limit on their performance.
As concrete examples, suspension seats are described hereinafter. The suspension seats are the seats available mainly in off-road vehicles, such as earth-moving machines or recreational vehicles (RV), or long-distance travelling vehicles, such as trucks or busses, and equipped with a vibration isolator mechanism. As the vibration isolator mechanism, metal springs, an air suspension, air dampers or the like are used. In these seats, the isolation of seat vibration has been improved within the frequency range from about 1.5 to 12 Hz, especially from 3 to 5 Hz. Therefore, suspension seats have a resonance frequency in the range of 1 to 2.5 Hz.
FIG. 51 depicts the vibration characteristics of conventional suspension seats. In FIG. 51, (a) indicates a rigid seat, (b) a suspension seat, (c) a spring-rich seat, and (d) a suspension seat having no damper.
In the seats with a low stiffness (i.e. soft ride comfort), there will be a large dynamic deflection when exposed to some shocks or to low frequency vibration. However, the travel of seat suspension mechanisms is usually limited to less than 100 mm so as not to interfere with the driver's operations such as, for example, depression of a pedal and the like. In the case of large dynamic deflections, it will cause the suspension seat to produce an end-stop impact.
In order to investigate the influences of the end-stop-impact on the performance of the suspension seat, Stiles performed a field survey of tractor driving in 1994. He found that 45% of suspension seats increased the acceleration levels experienced by the driver. He suggested that the end-stop impacts deteriorated the isolation efficiencies of the suspension seats. A shock absorber is used as a solution to a sudden or transient bump experienced by the vehicle.
Recently, an active suspension seat has been proposed wherein an actuator mounted to the seat works to active-control vibrations to enhance the ride comfort.
However, the vibration isolator mechanism employing the metal springs, air suspension, air dampers or the like cannot enhance the ride comfort or the feeling of use by decreasing a vibration frequency of 4-20 Hz from among vibrations transmitted through the vehicle floor.
Furthermore, the active suspension seat is heavy and expensive and is also required to always activate the actuator. If the actuator is turned off, vibrations are transmitted to a seat occupant through the actuator, thus losing the ride comfort.
On the other hand, in the suspension seat employing the shock absorber, if the damping force is too great, it may worsen the vibration isolation performance of the seat in the low and middle frequency region, i.e., at more than about 1.4 times of the resonance frequency.
The present invention has been developed to overcome the above-described disadvantages. It is accordingly an objective of the present invention to provide a magnetic spring having positive, 0- or negative damping characteristics by utilizing permanent magnets. Another objective of the present invention is to realize an inexpensive dynamic-characteristic control system or highly efficient engine of a simple construction by providing a stable nonlinear vibration mechanism or coefficient exciting vibration mechanism having the aforementioned magnetic spring and no physical damping structure.